In semiconductor device assembly, a semiconductor chip (or "die") may be bonded directly to a packaging substrate, without the need for a separate leadframe or for separate I/O connectors (e.g. wire or tape). Such chips are formed with ball-shaped beads or bumps of solder affixed to their I/O bonding pads. During packaging, the chip is "flipped" onto its active circuit surface in a manner forming a direct electrical connection between the solder bumps of the chip and conductive traces on a packaging substrate. Semiconductor chips of this type are commonly referred to as "flip chips", and are advantageously of a comparatively reduced size. For example, in current flip chip designs, the semiconductor die may be dimensioned as small as about 0.5.times.0.5 inch whereas the unbonded solder bumps arranged of a surface thereof may have a diameter on the order of 4 to 5 mils.
Briefly and as shown in FIG. 1, a prior art flip chip packaging assembly 10 may be constructed using conventional fabrication techniques. This packaging assembly 10 includes a semiconductor die 11 which is electrically interconnected to a packaging substrate 12 through solder joints (not shown). The die 11 is then mechanically mounted to the substrate 12 employing a cured layer 13 of underfill epoxy. This fabrication process, thus, produces a mechanically, as well as electrically, bonded chip assembly.
Semiconductor packages are typically subject to regular temperature cycling throughout their operational lifetime due to power dissipation in the form of heat. In order to improve the thermal performance and reliability of the flip chip packages, a thin, substantially plate-like heat spreader 15, typically composed of a high thermal conductivity metal, may be bonded to an inactive upper surface 16 of chip 11. A conventional heat spreader 15, such as that shown in FIGS. 1 and 2, is typically composed of a piece of flat, rigid thermally conductive metal, such as nickel-plated copper, about 0.50-0.65 mm thick.
Moreover, to promote rigidity of the flip-chip packaging assembly 10 so that the substrate will not warp during various fabrication processes or during operation, a thin, substantially flat plate-like stiffener ring 17 (FIGS. 1 and 3) is positioned adjacent to and peripherally about the semiconductor flip-chip die 11. As shown in FIG. 3, plate-like stiffener ring 17, which is typically about 0.50-0.65 mm thick, includes a rectangular annulus 18 dimensioned for receipt of the flip chip die 11 therethrough.
To mechanically bond the stiffener ring 17 and the heat spreader 15 to the packaging substrate 12, a thin bond layer 20, 21 of thermally conductive adhesive or epoxy is applied between the stiffener ring 17 and the packaging substrate 12, and between the stiffener ring 17 and the heat spreader 15. This adhesive is typically a thermo-set epoxy, such as is available from Hysol Corporation of Industry, California (product numbers 4511 and 4527), Ablestik Laboratories of Rancho Domingo, Calif. and Johnson Matthey Electronics of San Diego, Calif. Together, the thin epoxy layers 20, 21, the stiffener ring 17 and the heat spreader 15 cooperate to provide sufficient rigidity to the flip chip packaging assembly 10, as well as provide sufficient heat dissipation.
Optimum heat dissipation and therefore thermal performance of the flip chip packaging assembly 10, however, is best achieved by minimizing the thickness of the thermal bond-lines (i.e., thin bond layer 20, 21). Heat transfer from the chip to the heat spreader and stiffener are thus facilitated. Too thick a thermo-set epoxy layer will substantially impede effective heat conduction between the bonded components. The thermal bond-line thickness (preferably between about 1 mil (0.0254 mm) to about 11/2 mils (0.0381 mm)) obtained using the conventional apparatuses and methods described above is primarily dependent upon the flatness of both the chip, the heat spreader and the stiffener. Since these materials are both typically at least 0.50-0.65 mm thick, it is difficult to maintain their surfaces flat on a micron scale. These materials will typically warp from 25 to 50 microns (.mu.m) across the area of a typical chip, 125.0 mm.sup.2
Accordingly, considering the thickness of the stiffener ring 17 and the heat spreader 15 (i.e., about 0.50-0.65 mm), it is imperative that the opposed upper and lower planar surfaces 22, 23 and 24, 25 thereof be substantially flat. Any localized variations in the flatness of the planar surfaces on the order of more than about 3.2 mils (0.08 mm) may adversely affect the heat dissipation performance thereof. More importantly, such an uneven bond line thickness may cause delamination between adjacent bonded layers during component assembly or due to repeated operational thermal cycling. This delamination may then propagate throughout the entire layer should moisture become trapped therebetween.
Such flatness imperfections often originate at the stamping or punching operation employed to fabricate the metallic structure from a material web. For example, such metal movement often takes place at the corners 28 of the heat spreader 15 and the stiffener ring 17 due to the presence of respective mounting holes 26, 27 thereof (FIG. 4). Briefly, these mounting holes 26, 27, as shown at FIGS. 2 and 3, are formed and aligned for receipt of mounting posts (not shown) therethrough so that the flip-chip packaging assembly 10 may be clamped to a mounting surface.
Typically, problems result when the thickness between the mounting hole 26, 27 and the peripheral edges of these stamped components approach the thickness of the plate-like web material to be punched. Due to design criteria, it is desirable to position mounting holes 26 and 27 only about 0.70 mm from the peripheral edges, while the thickness of the stiffener ring 17 and the heat spreader 15 are preferably between about 0.50-0.65 mm thick. Accordingly, it is difficult to maintain the flatness of the upper and lower planar surfaces 22, 23 (FIG. 4) of these stamped devices within acceptable tolerances since subsequent stamping of the peripheral edges of these components often causes the portions near the mounting holes 26, 27 to cup out of plane. In fact, only about 20% of these fabricated components are within acceptable flatness tolerance, while about 80% of these components must be discarded. The cost of manufacture, thus, of the remaining acceptable components is substantially increased.